It is generally known that the thermal management of buildings constitutes a substantial portion of the total annual United States energy expenditures. These costs primarily consist of maintaining desirable ambient conditions and through thermal management and include interior heating and cooling, interior lighting, and interior privacy. To reduce both building energy demands and costs, considerable progress has been made in the design and development of improved thermal insulation, window glazing, heating and cooling systems, and lighting. However, radiative and convective heat transfer losses through windows remains as a major source of energy costs in building thermal maintenance.
Conventional building window designs offer the advantage of a natural lighting source but have numerous thermal management limitations since they are poor thermal insulators and offer little privacy. Improvements in conventional window designs have been achieved through the use of double and triple glazed glass for thermal insulation. Exterior or interior devices such as awnings, blinds and drapes are also employed to regulate the amount of light and heat transmitted through windows or provide for interior privacy. Such auxiliary devices provide protection against interior heating from sunlight in hot summer months or insulate against radiative and convective heat loss from interiors during cold winter months. However, the use of such auxiliary devices for interior thermal management frequently conflict with interior lighting requirements and thus must be accompanied by additional interior light sources due to the associated loss of a natural lighting source. Regardless of thermal management considerations, such devices are also necessary for maintaining interior privacy.
Control of radiative heating and cooling in buildings is less straightforward. FIG. 1 shows the spectral distribution of the sun's irradiance at sea level. Three relevant wavelength regions are indicated in the ultraviolet, the visible, and the infrared. Most window glasses absorb ultraviolet radiation so this portion of the spectrum can be disregarded. The visible region is important for both natural lighting and privacy offered by building windows. The infrared region is important for both summer heating and winter cooling of buildings. Windows which are highly transmissive in the visible provide good natural light but poor privacy. Windows which are highly transmissive in the infrared provide undesirable heating in the summer months and cooling in the winter months. Thus, the lighting, privacy, and thermal management requirements for a window are not always compatible and vary with both the time of day and the season. Therefore, there is a real need for building windows whose optical properties can be varied to match ambient lighting and heating conditions and requirements.
It is well known that the transmissivity and reflectivity of transparent materials can be varied by modifying their light absorption and light reflection characteristics. Thus, passive thermal management methods, such as applications of thin metal, glass, ceramic or polymer coatings or anti-reflection coatings, have been developed for increasing or decreasing the infrared reflectivity of transparent window materials such as glasses, ceramics and plastics. Similarly, the light absorption characteristics of transparent materials can be modified by either application of high absorptivity coatings, introduction of compositional additives which increase the intrinsic absorptivity of a transparent material, or the introduction of particulates having a high refractive index to increase internal light scattering.
It is also generally known that applications of semi-transparent, optically reflective coatings reduce radiative heating and cooling by reducing the transmission of infrared radiation through windows. For example thin films of silver, gold, copper, tin or zinc oxide have been successfully employed to reflect or absorb a substantial portion of solar radiation. While conventional reflective coatings may be useful in reducing building cooling requirements during the summer and heating requirements during the winter, they typically produce a significant reduction in natural light transmission at visible wavelengths. In addition, these coatings generally do not provide for interior privacy and must be used with other devices such as blinds and drapes. An additional limitation of such reflective coatings is that they are passive films and windows utilizing such films are thus restricted to fixed reflection and transmission characteristics which cannot be adjusted to respond to climatic conditions or daily light cycles to meet interior heating and lighting requirements.
A major technical limitation of many conventional reflective coatings in building and vehicle window applications is that they tend to have a characteristic near infrared optical absorptivity which produce both a reduction in thermal transfer efficiency, due to secondary re-radiation from windows, and generates daily and seasonal thermal cycling from solar radiation absorption, resulting in cyclic thermal expansion stresses which can produce premature fatigue failure of the window glass [see R. Campbell, "Cracking Riddle of Hancock Windows", Boston Globe, Apr. 9, 1996]. To avoid such problems, it is necessary to identify optical materials whose near infrared solar radiation absorptivity is less than 50%.
More recently, methods have been developed which provide for variably adjusting the reflectivity and absorption characteristics of transparent materials. Active methods and devices have been developed which provide for optical switching between a dark, or colored, state and a transparent, or bleached state. These devices provide for adjusting the transmission characteristics of a transparent material to match the required lighting conditions.
For building and automobile applications, a spectrally selective transmission modulator must meet certain optical, mechanical, and chemical requirements. Optically, the visible transmission in the bleached state should be at least 70%, particularly in vehicle applications where adequate night vision is required for safety. The transmission in the colored state should be less than 5% to ensure privacy and security at night. The device must be free of visual defects such as discoloration and translucence. The device must have a low absorptivity to reduce heating and resultant thermal stresses induced by absorption of solar radiation. Additionally, the switching times between colored and bleached states must meet application requirements ranging from seconds for vehicles to minutes for buildings. Mechanically, the optically modulated window must be stable to large variations in temperature, repetitive thermal cycling and mechanical stresses induced by repetitive insertion and depletion of cations within the electrochromic material which produce volumetric changes in crystal lattices. Chemically, the device must be resistant to condensation, photochemical bleaching, and large variation in humidity.
One alternative to conventional passive coatings and auxiliary devices which meets these optical, mechanical and chemical requirements for thermal and visible radiation management has been the development of active electrochromic devices and windows. Electrochromic windows are large-area, multi-layered, thin film electronic devices whose optical properties can be modulated by application of an electric current at low voltages. These devices function as tunable band pass filters whose transmission response may be adjusted to match ambient light conditions and requirements. Such devices generally consist of multi-layered, thin film coatings applied to transparent substrates in which at least one thin film layer is comprised of an electrochromic material which is responsible for their optical property modulation. A device with such variable spectral selectivity can transmit or reflect a specific spectral region of solar radiation or thermal infrared radiation depending on the thermal and ambient light conditions and requirements.
The use of electrochromic devices which provide for modulation of reflectivity and transmissivity in transparent materials is particularly useful in applications involving architectural or building windows, vehicle windows or windshields, rear view mirrors, aviation visors and sunglasses, where adjustments in the optical properties of transparent materials are required to match diverse ambient lighting conditions.
Electrochromic devices and windows offer a number of distinct advantages over conventional methods for thermal radiation management. They provide for transmission of substantially all visible solar radiation but reflect substantially all of the thermal infrared radiation from either solar radiation or interior building radiation. They thus provide for reduction in cooling requirements in summer months and reduced heating requirements in winter months. In addition, they provide for adjustment of visible transmission through a window to match interior privacy needs to ambient light conditions. Due to their low optical absorptivity, electrochromic devices do not suffer from the same cyclic fatigue problem encountered with conventional passive coatings and have improved durability with daily and seasonal optical cycling. Prototype devices have been repetitively cycled between a colored and bleached state for tens of thousands of cycles with consistent and stable optical properties. An additional advantage of electrochromic devices is that they eliminate the need for auxiliary devices such as awnings, blinds, and drapes for thermal or light transmission management through building windows.
Electrochromic devices which demonstrate cyclic and reversible reflectivity modulation are known in the art. For example, see U.S. Pat. No. 4,889,414 to Rauh and Goldner, U.S. Pat. No. 4,902,110 to Green, U.S. Pat. No. 5,260,821 to Chu, et al, U.S. Pat. No. 5,202,788 to Weppner, et al, and U.S. Pat. No. 5,455,126 to Bates, et al. A cross-sectional schematic of a typical prior art device is shown in FIG. 2. A typical transmission and reflectivity spectra of a prior art device is shown in FIG. 3a and 3b. In general, a typical prior art device has a relative high transmission in the bleached state, close to 70%,a low colored-state transmission in the visible, less than 20%, and reasonable switching times. As shown by FIG. 3b, the reflectivity of a typical prior art device in the colored state is less than 50% with an undesirable high absorption in the near infrared. An additional undesirable limitation of prior art devices is a low transmission modulation in the infra-red with a typical reflectivity modulation of between 30 to 40%. In order for such electrochromic devices to gain widespread commercial acceptance in building and vehicle window applications, colored-state reflectivity of at least 60% and colored-state absorptivity of less than 50% in the near infrared are required with a reflectivity modulation of at least 50%.
In addition to non-optimum reflectance and absorption characteristics of current electrochromic devices, commercial applications of these devices are further hampered due to certain electronic, mechanical and fabrication limitations. Due to the complexity of design and fabrication methods with a multi-layered electrochromic device, electron and ion leakage or shorts limit the optical modulation and lifetime of current devices. Additionally, substantial mechanical stresses are induced due to lattice distortion from volumetric expansion and contraction associated with ion insertion and extraction from the anode and cathode layers. Furthermore, due to high temperature deposition methods used to fabricate these devices, thermally induced stresses generated during device processing typically cause cracking within and between device layers and lead to poor yields in manufacturing. Thus, innovative electrochromic device designs, thin film materials, and methods for fabricating the same are necessary for optimizing device performance and facilitating commercialization of electrochromic devices in window applications.